Regulation of Sterol Biosynthesis in the Human Fungal Pathogen Aspergillus fumigatus: Opportunities for Therapeutic Development

Abstract

Sterols are a major component of eukaryotic cell membranes. For human fungal infections caused by the filamentous fungus Aspergillus fumigatus, antifungal drugs that target sterol biosynthesis and/or function remain the standard of care. Yet, an understanding of A. fumigatus sterol biosynthesis regulatory mechanisms remains an under developed therapeutic target. The critical role of sterol biosynthesis regulation and its interactions with clinically relevant azole drugs is highlighted by the basic helix loop helix (bHLH) class of transcription factors known as Sterol Regulatory Element Binding Proteins (SREBPs). SREBPs regulate transcription of key ergosterol biosynthesis genes in fungi including A. fumigatus. In addition, other emerging regulatory pathways and target genes involved in sterol biosynthesis and drug interactions provide additional opportunities including the unfolded protein response, iron responsive transcriptional networks, and chaperone proteins such as Hsp90. Thus, targeting molecular pathways critical for sterol biosynthesis regulation presents an opportunity to improve therapeutic options for the collection of diseases termed aspergillosis. This mini-review summarizes our current understanding of sterol biosynthesis regulation with a focus on mechanisms of transcriptional regulation by the SREBP family of transcription factors.

Keywords: Aspergillus fumigatus; SREBPs; antifungal agents; ergosterol; triazoles.

FIGURE 1

The canonical fungal ergosterol biosynthetic pathway. Biosynthesis of ergosterol from Acetyl-coA depicting intermediate steps and enzymes catalyzing the intermediate reactions. Known regulators of Erg genes are shown on right side, whereas anti-fungal agents targeting various pathway steps are shown on the left of the pathway. ^∗Signifies the pathway differences in Aspergillus fumigatus and Saccharomyces cerevisiae. In S. cerevisiae, Cyp51 converts lanosterol into C4 methylated sterols which are demethylated by Erg24 and Erg25 into zymosterol. Erg6 then converts zymosterol into fecosterol.

FIGURE 2

Proposed Model of SREBP regulation in A. fumigatus. SrbA (full length SrbA protein is represented as N terminus and C terminus joined by a transmembrane region) is an ER resident protein, however, the membrane topology of SrbA is unknown. Unlike Schizosaccharomyces pombe, SCAP has not been identified in A. fumigatus, and it is not clear if an unidentified ER resident protein “X” forms a complex with SrbA to regulate SrbA activation. Golgi resident Dsc proteins (DscA-E collectively known as DSC complex) are indispensable for SrbA cleavage and activation, however, it is unclear if there is anterograde and/or retrograde movement of SrbA to the Golgi or Dsc complex movement to the ER for SrbA cleavage. The rhomboid protease RbdB is indispensable for A. fumigatus cleavage and in S. pombe Rbd2 interacts with the UBX domain of Dsc5 (DscE homolog of A. fumigatus) via Cdc48. However, the nature of this interaction needs validation in A. fumigatus. The Signal Peptide Peptidase (SppA) is an ER resident aspartyl protease involved in regulated intramembrane proteolysis and is indispensable for SrbA cleavage, however, it is not clear if SppA cleavage is preceded or followed by action of the Dsc complex and/or RbdB mediated cleavage. Once cleaved, the C terminus of the protein is potentially degraded and the N terminus translocates to the nucleus where it binds to SRE elements in the promoter region of genes involved in the hypoxia response. SrbA also positively regulates its own mRNA levels by binding the SRE element in the promoter region of srbA. Solid lines depict experimentally validated results, whereas dotted lines indicate predicted but not experimentally tested mechanisms. X – Unknown ER resident protein or unidentified SCAP (like) homolog.

FIGURE 3

Regulation of cyp51A expression in A. fumigatus. In azole susceptible isolates, two SRE elements and one CBC binding motif is present in the promoter region of cyp51A. Binding of SrbA to SREs positively regulates cyp51A expression while binding of the CBC to the CGAAT motif negatively regulates expression. Azole challenge (arrow) also positively regulates cyp51a expression. In strains containing either a TR34 promoter repeat or TR46 promoter repeat, SRE elements and CBC binding motifs are duplicated and SrbA effectively binds SREs in the duplicated region, thereby increasing cyp51A expression. Mutation in the HapE (P88L) subunit of the CBC lowers the affinity of the CBC to the CGAAT motif thereby inhibiting negative regulation of cyp51A expression. Solid lines depict experimentally validated results, whereas dotted lines indicate hypotheses which need further validation. Importantly, additional regulatory factors are likely in play at this important gene promoter.